1. Field of the Invention
This invention relates to a composite composition comprising granular sodium percarbonate with better stability in storage and fast solubility in water to be used by itself or as a composition of a bleaching agent.
2. Description of the Prior Art
The use of sodium percarbonate and sodium perborate in powder form have been used for many years as bleaching agents for clothes. When used as a beaching agent, sodium perborate requires an activator. Due to recent trends in environmental and energy conservation, there has been a demand for the use of sodium percarbonate which is readily soluble in cold water and does not require an activator. However, in order to be an affective bleaching agent, it is critical that sodium percarbonate not decompose during its delivery or storage, not lose any active oxygen, and quickly dissolve in water.
A highly soluble form of sodium percarbonate is desirable in order to effectively wash cotton-based clothes. In a typical washing scenario, cotton based clothes, white clothes and dirty socks, are pre-soaked in water prior to washing. Sodium percarbonate is added to cold water together with a detergent and the clothes and left for a couple of hours prior to washing. If the sodium percarbonate particles can be readily dissolved in the wash water, damage or discoloration of the clothes may be prevented. Additionally, if the time needed to solubilize sodium percarbonate in unagitated water is shortened, the overall wash time may be shortened.
In general, smaller particles are more readily dissolved in water, therefore selection of a smaller particled, micro-powder form of sodium percarbonate may shortened its solubilization time. However, in detergents containing zeolite, the zeolite significantly reduces the stability of sodium percarbonate. A sodium percarbonate micro-powder mixed with zeolite decomposes very quickly and makes it impossible to use sodium percarbonate as a component of a bleaching detergent. To lessen the decomposition affect, commercially available sodium percarbonate is in granular form instead of micro-powder. Accordingly, the granular sodium percarbonate's solubility in water is reduced.
To solve these problems, many inventions related to the stabilization and composition of sodium percarbonate have been disclosed for the past 10 years. Among these inventions, European patent No. 567,140 discloses a novel stabilization method in which the surface of sodium percarbonate particles are coated with boric acid or borate. However, this patent has disadvantages in that stability was not as good as expected, and the solubility rate in water was far slower than that of the conventional product.
One conventional method to increase the solubility rate of sodium percarbonate in water is the thermal treatment of sodium percarbonate as disclosed in U.S. Pat. No. 3,953,350.
Sodium percarbonate is decomposed at temperatures of more than 120.degree. C. to generate hydrogen peroxide, its primary decomposition product. Further, in parallel with the continuous decomposition, oxygen and water are generated as a final decomposition product, as disclosed in The Journal of Japan Industrial & Chemical Association (Vol. 47, No. 2, 1976, pp 84-90). These gaseous molecules remain trapped in the sodium percarbonate crystal lattice and are released when the sodium percarbonate is dissolved in water. This imparts a foaming action to the sodium percarbonate particles when they are dissolved in water. However, this method has proven to be uneconomical in that micro-powder is generated during the thermal treatment of sodium percarbonate, and the loss of active oxygen is inevitable.
Japan Unexamined Patent Application No. Hei 9-227108 discloses a process to improve the stability and the solubility of sodium percarbonate when it is blended with detergents by coating the surface of sodium percarbonate particles. A description of the process is outlined below.
A binder is added to wet sodium percarbonate prepared by the wet process method. The binder and sodium percarbonate mixture is then granulated. On average, particle sizes of up to 800 .mu.m are formed. Next, the mixture is dried and coated again using fluidized-bed technology. Coating materials include: amino acid derivatives, aliphatic, aromatic polycarboxylic acid and the salts thereof. The object of the invention recited in Japan Unexamined Patent Application No. Hei 9-227108) is to produce large particles of sodium percarbonate. However, in light of the adverse solubility effects associated with larger, granulated sodium percarbonate particles, it is questionable whether improved solubility is achieved in this process. Furthermore, this Japanese invention requires processing steps for the manufacture, fabrication and coating of the sodium percarbonate particles. The processing steps are very complicated resulting in enormous production costs.